The amount of mercury required to be enclosed in the lamp desirably is as small as possible from the viewpoint of environmental protection. Therefore, it is required that a minimum amount of mercury should be enclosed in a glass bulb with high precision.
However, mercury has a high surface tension, which makes it difficult to measure off a small amount of the same accurately. Besides, since it tends to adhere to a wall of a discharge thin tube or the like, the loss of mercury occurring during enclosure is considerable. Therefore, conventionally, an amalgam of mercury in a pellet form has been enclosed in place of pure mercury.
For instance, the patent document 1 discloses a fluorescent lamp in which an amalgam containing mercury and zinc as principal components (hereinafter referred to as ZnHg) is enclosed. The patent document 2 discloses a fluorescent lamp in which an amalgam containing mercury and tin as principal components (hereinafter referred to as SnHg) is enclosed.
Problems have arisen in the configurations with ZnHg and SnHg, respectively. The problem with the configuration with ZnHg is that in a manufacturing process, when an amalgam pellet is brought into a heated glass bulb, an amount of mercury vapor released from the amalgam pellet is small. It is known generally that upon the first lighting of the fluorescent lamp, mercury vapor is consumed rapidly due to physical adsorption onto an internal wall of the glass bulb or chemical reaction with a phosphor-film-forming material or an impurity gas, causing the mercury vapor level to tend to be insufficient. Further, recently, with a view to improving the lamp efficiency, the internal diameter of the glass bulb tends to decrease while the discharge path tends to increase, thereby making it difficult to cause mercury vapor to spread throughout the glass bulb, which makes the mercury vapor level more insufficient. If a fluorescent lamp is left to be on for a long time in such a state of insufficient mercury vapor, lighting defects such as non-lighting or flickering occur, or a circuit for lighting is overloaded. Therefore, problems of lighting defects, etc. tend to occur in a fluorescent lamp in which ZnHg is used.
On the other hand, the problem lying in the configuration with SnHg is that an amalgam pellet is heavy. Since the mercury content of SnHg is smaller than that of ZnHg, when SnHg is used, the weight of an amalgam pellet has to be increased further so as to enclose the same amount of mercury as that in the case of ZnHg. If an amalgam pellet is heavy, the amalgam pellet tends to cause the phosphor film to peel off when it collides against a phosphor film due to vibration during transportation or the like, thereby impairing the appearance of the fluorescent lamp, etc.
It should be noted that an amalgam pellet of SnHg is stable in the case where the mercury content therein is in a range of 15.8 wt % to 29.7 wt %, and in the case where the mercury content is set to be more than that, mercury leaks out of the amalgam pellet in some cases. Therefore, it is difficult to increase an amount of enclosed mercury by increasing the mercury content in the pellet.    Patent document 1: JP 3027006 B    Patent document 2: JP 2000-251836 A